The invention concerns a method and a device for manipulating microscopic particles, especially for manipulating particles in a plasma-crystalline state.
It is known that microscopic solid particles in a plasma may be oriented in a macroscopically regular arrangement as so-called plasma crystal. The properties of plasma crystals are for instance described by H. Thomas et al. in xe2x80x9cPhys. Rev. Lett.xe2x80x9d, Volume 73, 1994, page 652 ff., or by H. Thomas and G. E. Morfill in xe2x80x9cNaturexe2x80x9d, Volume 379, 1996, page 806 ff.
A quantitative description of plasma crystals on the basis of molecular-dynamic simulations of Yukawa systems and delimitation with respect to xe2x80x9cliquidxe2x80x9d states is described by S. Hamaguchi et al. in xe2x80x9cPhysical Review Exe2x80x9d, Volume 56, 1997, p. 4671 ff. This publication was published after the priority date of the present application. The delimitation between a plasma-crystalline and a non-plasma-crystalline (for instance liquid) state is performed on the basis of a phase diagram, whose abscissa is formed by a dimensionless parameter xcexa as quotient from the charge-dependant distance between particles and the so-called Debye length and its ordinate is formed by a parameter xcex93, which dimensionless describes the coulomb interaction of the particles. Because the abscissa and ordinate parameters depend on the operating parameters of the plasma, therefore changes in state of the plasma states of the particles may be achieved by changes in operating parameters.
Important aspects of plasma crystal formation will hereinafter be explained with reference to a conventional arrangement for formation of a plasma crystal according to FIG. 14.
In a plasma state, which is for instance created by glow discharge or gas discharge, a gas includes differently charged particles, like positively or negatively charged ions, electrons and radicals, but also neutral atoms. If there are microscopic particles in the plasma (order of magnitude: xcexcm), for instance dust particles, then these are electrically charged. The charge may be up to some hundred thousands of electron charges, depending on the particle size and the plasma conditions (gas type, plasma density, temperature, pressure, etc.). Under suitable particle and plasma conditions, coulomb forces are generated between the charged particles, under which effect the particles take a plasma crystalline state as a two or three dimensional arrangement. Besides the coulomb forces, an energy reduction at the particles by collision with neutral atoms within the plasma has an effect.
An arrangement for formation of plasma crystals is by example shown in FIG. 14 (also see the above mentioned publication in Phys. Rev. Lett.). In a reactor (vessel walls not shown) with a carrying gas, two plane discharge electrodes are arranged one over the other. The lower circular or disc-shaped HF electrode 11 is fed with an alternating voltage, and the upper, ring-shaped counterelectrode 12 is for instance grounded. The distance between the electrodes amounts to about 2 cm. A control circuit 13 is installed for connecting the HF generator 14 to the HF electrode 11 and to feed the grounding and separating circuit 15 of the counterelectrode 12. The high frequency energy may for instance be coupled in at a frequency of 13.56 MHz and a power of about 5 W. The carrying gas is formed by inert gases or reactive gases under a pressure of about 0.01-2 mbar. By means of a dust dispenser (not shown), dust particles are introduced into the reactor. The dust particles arrange themselves as a plasma crystal in a balanced condition, in which the gravitation force G effecting the particles are counterbalanced by the electrical field force E, which has an effect on the particles near the HF electrode 11 depending on their charge. If this is a mono dispersed dust grain distribution, then the plasma crystal arrangement is either performed as a mono layer in a plane, or as a multi-layer state when forming 3-dimensional plasma crystals. The plasma crystal is detectable by the naked eye under light up to a particle size of about 1 xcexcm. The visibility of the plasma crystal is improved by a helium-neon laser 16, arranged laterally, whose beam is fanned out to a diameter of about 150 xcexcm to the size of the lateral crystal dimension using a cylinder lens combination 16a. Observation of the plasma crystal is performed using a CCD camera 17, which is fitted with an enlarging macro optics 18 and controlled by image processing 19, which is also connected to the laser 16.
The behaviour of microscopic particles in plasma is of great theoretical and practical interest. The theoretical interest especially concerns the plasma crystals and their change of state. The practical interest is derived from the fact that plasma reactors employed for coating or processing procedures (especially in semiconductor technology) have an electrode structure according to FIG. 14.
In prior arrangements for examination of plasma crystals, the means for influencing the plasma crystals were limited to the type of particles used and the plasma conditions realized. A means for deliberate and location-selective handling of plasma crystals is currently not available, so that up to now no practical use for plasma crystals was known.
An object of the invention is to provide a method for manipulating particles in plasma, especially for influencing particles themselves or for modification of a substrate surface and a device for realizing the method.
The invention is based on the following basic findings. The properties of a plasma crystal, especially the geometric shape, does not only depend on the properties of the plasma more over or the particles. Moreover, it is possible to modify the shape of a plasma crystal, especially the shape of the outer edge or the cross sectional shape, by a location-selective effect on the above mentioned balance between gravitational forces and electrical forces. For this purpose, the external forces having an effect on the particles, for instance by a location-dependent change of a static, quasi-static or low frequency changing electrical field between the electrodes of a plasma reactor are varied by location-selective particle discharge or by location-selective particle irradiation (effect of adjusting forces). In this manner, particles in a plasma may be arranged on any curved plane with any edge in a plasma-crystalline state. The particles in the plasma may therefore be moved in a predetermined manner, whereby this movement is reversible, so that the plasma-crystalline state may even be switched between different shapes.
Another important aspect of the invention consists of the fact that by location-selective deformation of a plasma crystal, different parts of the plasma crystal are subject to different plasma conditions. This especially enables, in a plasma between two essentially plane electrodes, location-selective plasma treatment of parts of the plasma crystal (for instance coating or ablation). Such a location-selective particle treatment may be followed by deposition on a substrate.
Furthermore, an important aspect of the invention consists of the fact that formation of a plasma-crystalline state remains uninfluenced by the presence of a substrate in a plasma reactor, especially between reactor electrodes for creation of a glow discharge or gas discharge. It is especially possible to perform the above mentioned switching processes in the immediate vicinity of an areal, plane or curved substrate and subsequently reduce the distance between the particles in a plasma-crystalline state and the substrate surface in such a manner that at least a predetermined part of the particles is applied to the substrate surface. The reduction of the distance may be performed either by influencing the field forces holding the particles in position or by movement of the substrate surface. Therefore particles in a plasma-crystalline state may be deposited on substrate surfaces in patterns of any design. Therefore, the invention provides for a new, location-selective, mask-free coating Method creating modified surfaces. Due to the particles applied, the modified surfaces have changed electronic, optical and/or mechanical properties. But it is also possible to use the location-selectively applied particles themselves for masking or conditioning of the substrate surface before a subsequent further coating step.
A device according to the invention for manipulating of particles in plasma-crystalline state includes a reaction vessel containing devices for forming a plasma and at least one substrate. The devices for forming the plasma are preferably formed by planar, essentially parallel electrodes, in whose distance the substrate is movable. The electrodes within the reaction vessel may have field-shaping structures for location-selective influence of particles in plasma-crystalline state. The reaction vessel may furthermore contain means for location-selective particle discharge (for instance UV exposure means with a masking device), means for exerting radiation pressure on the particles, observation means and a means of control.
A specific aspect of the invention is the design of the electrodes for location-selective influencing of the particles within the reaction vessel. According to the invention, an electrode device (or: adaptive electrode) is provided, which has a plurality of electrode segments, which are simultaneously fed with a high frequency voltage and in each case separately with a specific direct voltage or low frequency voltage. The high frequency voltage is adapted for the purpose of creating respectively preserving a plasma state within the reaction vessel, while the direct respectively low frequency voltage is adapted for creating a static or slowly changing field distribution within the reaction vessel, under whose effect the particles arrange or move within the reaction vessel. Further important characteristics of the adaptive electrode are formation of a matrix formed of miniaturized electrode segments (point electrodes), design of the matrix as essentially planar, layered component, whose electrode side points at the reaction vessel and whose back bears control electronics, pressure relief of the component, for instance by creation of a vacuum in the space to which the back of the electrode device points, and provision of a tempering device for control electronics.